Abstract

Background

The Dmbx1 gene is important for the development of the midbrain and hindbrain, and mouse gene
targeting experiments reveal that this gene is required for mediating postnatal and
adult feeding behaviours. A single Dmbx1 gene exists in terrestrial vertebrate genomes, while teleost genomes have at least
two paralogs. We compared the loss of function of the zebrafish dmbx1a and dmbx1b genes in order to gain insight into the molecular mechanism by which dmbx1 regulates neurogenesis, and to begin to understand why these duplicate genes have
been retained in the zebrafish genome.

Results

Using gene knockdown experiments we examined the function of the dmbx1 gene paralogs in zebrafish, dmbx1a and dmbx1b in regulating neurogenesis in the developing retina and midbrain. Dose-dependent loss
of dmbx1a and dmbx1b function causes a significant reduction in growth of the midbrain and retina that
is evident between 48-72 hpf. We show that this phenotype is not due to patterning
defects or persistent cell death, but rather a deficit in progenitor cell cycle exit
and differentiation. Analyses of the morphant retina or anterior hindbrain indicate
that paralogous function is partially diverged since loss of dmbx1a is more severe than loss of dmbx1b. Molecular evolutionary analyses of the Dmbx1 genes suggest that while this gene family is conservative in its evolution, there
was a dramatic change in selective constraint after the duplication event that gave
rise to the dmbx1a and dmbx1b gene families in teleost fish, suggestive of positive selection. Interestingly, in
contrast to zebrafish dmbx1a, over expression of the mouse Dmbx1 gene does not functionally compensate for the zebrafish dmbx1a knockdown phenotype, while over expression of the dmbx1b gene only partially compensates for the dmbx1a knockdown phenotype.

Conclusion

Our data suggest that both zebrafish dmbx1a and dmbx1b genes are retained in the fish genome due to their requirement during midbrain and
retinal neurogenesis, although their function is partially diverged. At the cellular
level, Dmbx1 regulates cell cycle exit and differentiation of progenitor cells. The
unexpected observation of putative post-duplication positive selection of teleost
Dmbx1 genes, especially dmbx1a, and the differences in functionality between the mouse and zebrafish genes suggests
that the teleost Dmbx1 genes may have evolved a diverged function in the regulation of neurogenesis.